linux-kernel - Re: [RFC PATCH 1/2] mm, memory_hotplug: drop artificial restriction on online/offline

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Date:   Thu, 25 May 2017 08:28:08 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Reza Arbab <arbab@...ux.vnet.ibm.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] mm, memory_hotplug: drop artificial restriction
 on online/offline

On Wed 24-05-17 16:50:56, Reza Arbab wrote:
> On Wed, May 24, 2017 at 02:24:10PM +0200, Michal Hocko wrote:
> >74d42d8fe146 ("memory_hotplug: ensure every online node has NORMAL
> >memory") has added can_offline_normal which checks the amount of
> >memory in !movable zones as long as CONFIG_MOVABLE_NODE is disable.
> >It disallows to offline memory if there is nothing left with a
> >justification that "memory-management acts bad when we have nodes which
> >is online but don't have any normal memory".
> >
> >74d42d8fe146 ("memory_hotplug: ensure every online node has NORMAL
> >memory") has introduced a restriction that every numa node has to have
> >at least some memory in !movable zones before a first movable memory
> >can be onlined if !CONFIG_MOVABLE_NODE with the same justification
> >
> >While it is true that not having _any_ memory for kernel allocations on
> >a NUMA node is far from great and such a node would be quite subotimal
> >because all kernel allocations will have to fallback to another NUMA
> >node but there is no reason to disallow such a configuration in
> >principle.
> >
> >Besides that there is not really a big difference to have one memblock
> >for ZONE_NORMAL available or none. With 128MB size memblocks the system
> >might trash on the kernel allocations requests anyway. It is really
> >hard to draw a line on how much normal memory is really sufficient so
> >we have to rely on administrator to configure system sanely therefore
> >drop the artificial restriction and remove can_offline_normal and
> >can_online_high_movable altogether.
> 
> I'm really liking all this cleanup of the memory hotplug code. Thanks!  Much
> appreciated.

I am glad to hear that and more is to come.

> Acked-by: Reza Arbab <arbab@...ux.vnet.ibm.com>

Thanks!
-- 
Michal Hocko
SUSE Labs